We propose to continue the study of mechanisms of repair of UV-damaged DNA in two bacterial species, Escherichia coli and Bacillus subtilis. In particular, we plan to extend our studies of post-replication repair in excision-deficient cells, using a dimer-specific endonuclease to determine the fate of dimers during repair. We are especially interested in the origin and significance of high molecular weight daughter DNA apparently copied from irradiated template molecules which still contain UV-induced dimers. We wish to determine whether the production of this DNA reflects a rapid filling of gaps in discontinuous daughter strands synthesized after irradiation, or some other repair mechanism, perhaps a dimer by-pass.